1. Field of the Invention
The present invention relates to an electronic musical instrument which creates a musical sound which has a touch response characteristic corresponding to the key striking force.
2. Description of the Prior Art
In conventional electronic musical instruments such as an electronic organ, a synthesizer and so forth, a variety of touch response systems have been proposed for detecting performance information such as the key striking velocity, pressure and impact force. For example, one of the conventional systems is, for example, to detect, as touch response information, the key striking pressure by means of a piezoelectric element, pressure-sensitive element or like pressure sensor provided for each keyboard. But this system has the defects of substantial variations in the output analog quantities of the individual sensors, difficulties in the detection of pressure and high cost.
As a system for detecting the key striking velocity through utilization of a time constant circuit which is made up of a resistance element and a capacitance element, there has been proposed a system wherein charges stored in the capacitive element are discharged only for a period of time corresponding to the time difference between status changes of two switches which are mounted on a keyboard and temporally actuated, thereby detecting an exponential decrease in the voltage across the capacitance element. But this system encounters difficulties in equally setting individual time constants and calls for a relatively large capacity capacitance element, and hence is not suitable for microfabrication as an LSI or the like.
Another conventional system has been proposed which measures the time interval only between the status changes of the abovesaid two temporally-actuated switches by means of time measuring circuits respectively corresponding to individual keyboards, but this system is very expensive. As a modification of the technique for detecting the depressed key state through use of a microprocessor (hereinafter referred to as a CPU), there has been proposed a system which scans the status changes of the two switches for each key by the CPU to detect the time difference between their status changes by means of software, or by an external hardware arrangement, but this system has the shortcoming that a touch response characteistic of sufficient resolution cannot be obtained owing to a limitation on the processing speed rate of the CPU.
Another conventional system is one that is intended to increase the substantial processing speed of the CPU by assigning time measurement proseccing channels smaller in number than the key boards used at the moment of detection of a change in the switch status, but this system is defective in that the number of tones to be produced simultaneouly and the tone generator assignement method are limited in terms of hardware.
Another conventional system is one that employs a data area corresponding to each keyboard and a counter to be used on a time-shared basis and counts only the time interval occurring between status changes of the abovementioned two temporally actuated switches. With this system, however, since the counter output has no such a natural temporal variation curve as is obtainable with the aforesaid time constant circuit system, it is necessary to use a circuit which performs a data translation and refers to a data translation table.
As regards chattering inherent in keyboard switches, a conventional masking method by a software timer of a CPU scanning circuit which detects only the ON-OFF state of switches cannot be used in terms of processing speed. It is therefore necessary to provide a chattering preventing circuit in a hardware form for each keyboard. A touch response characteristic of satisfactory accuracy cannot be obtained without such chattering preventing means, but the provision of such means will inevitably increase the manufacturing costs.
In general, a touch response detect operation processing block and a digital musical tone generation processing block perform optimum actions at entirely different timing. In this case, it is possible to employ a handshaking system or buffer memory for transferring touch response information, but much time is consumed for the transfer process of each touch response information. This problem can be solved by simultaneous actuation of the both blocks, but since the amount of data to be processed varies with the state of performance, either one of the blocks is always placed in the wait state, so the efficiency of processing is low.
In a touch response operation processing system it has been proposed to efficiently carry out a touch response operation in units of eight or 16 bits for one word in view of limitations on the number of bits of a RAM or CPU. With one word-eight bits, however, a touch response characteristic of sufficiently high accuracy cannot be obtained. With one word-16 bits, the accuracy of the touch response characteristic is high but the manufacturing costs increases very much.